By the spread of mobile devices such as mobile phones and laptop computers, the role of a secondary battery as a power source thereof is regarded as important. The requirement is that this secondary battery have a small size, a light weight, and a high capacity, and that the deterioration of the charge and discharge capacity be difficult to occur. As a secondary battery having such properties, a lithium ion secondary battery is now widely used.
In the negative electrode of the lithium ion secondary battery, carbon such as graphite or hard carbon is mainly used. By using the carbon, the charge and discharge cycle can be repeated well, but capacity near the theoretical capacity has already been realized. Therefore, the large capacity improvement cannot be expected in the future. On the other hand, since the demand for capacity improvement in lithium ion secondary batteries is strong, there are some researches regarding a negative electrode material which has a capacity higher than that of the carbon, in other words, which has high energy density.
As the negative electrode of the lithium ion secondary battery, there are also some studies regarding metal lithium from the standpoint of high energy density and light weight. However, if metal lithium is used for the negative electrode, a dendrite (dendritic crystal) is precipitated on the surface of the metal lithium during charge and the crystal penetrates through the separator which causes an internal short-circuit, and thereby the life time of the battery may be shorten.
There are some studies in which a Li absorbing substance represented by a composition formula of LixA (A is an atom such as silicon or tin) that is alloyed with lithium is used as the negative electrode active material in order to raise the energy density. This Li absorbing substance can absorb and desorb a large amount of lithium ion with respect to the unit volume and has a high capacity.
As a method for improving the cycle property in addition to raising the battery energy density, Patent documents 1 and 2 disclose a method that uses a negative electrode active material which contains silicon and silicon oxide.
Also, in the silicon negative electrode, a method for providing a recessed portion on the coin-type molded negative electrode and for forming a crack which starts from the recessed portion along the thickness direction thereof is proposed in Patent document 3. According to the method disclosed in Patent document 3, by forming a crack in the molded negative electrode, an effect is produced in which the disconnection of the current collecting paths along the thickness direction of the negative electrode is alleviated.